A classic tiling game. print a board, 1 cross piece, and several "L" pieces. place the cross on one random spot and try to place "L" tiles to cover all the remaining spots.
the 4x4 and 7x7 can always be completed no matter where the cross is placed. The 5x5 board can be solved only on some configurations.

The Scad file can be configured to produce other side boards.